λ Bootis stars with composite spectra
We examine the large sample of λ Boo candidates collected inTable 1 of Gerbaldi et al. (\cite{Gerbaldi2003}) to see how many of themshow composite spectra. Of the 132 λ Boo candidates we identify22 which definitely show composite spectra and 15 more for which thereare good reasons to suspect a composite spectrum. The percentage ofλ Boo candidates with composite spectra is therefore >17% andpossibly considerably higher. For such stars the λ Booclassification should be reconsidered taking into account the fact thattheir spectra are composite. We argue that some of the underabundancesreported in the literature may simply be the result of the failure toconsider the composite nature of the spectra. This leads to thelegitimate suspicion that some, if not all, the λ Boo candidatesare not chemically peculiar at all. A thorough analysis of even a singleone of the λ Boo candidates with composite spectra, in which thecomposite nature of the spectrum is duly considered, which woulddemonstrate that the chemical peculiarities persist, would clear thedoubt we presently have that the stars with composite spectra may not beλ Boo stars at all.Based on observations collected at ESO (Echelec spectrograph) and at TBL(Telescope Bernard Lyot) of the Pic du Midi Observatory (France).
|
The heterogeneous class of lambda Bootis stars
We demonstrate that it is arduous to define the lambda Boo stars as aclass of objects exhibiting uniform abundance peculiarities which wouldbe generated by a mechanism altering the structure of their atmosphericlayers. We collected the stars classified as lambda Boo up to now anddiscuss their properties, in particular the important percentage ofconfirmed binaries producing composite spectra (including our adaptiveoptics observations) and of misclassified objects. The unexplained RVvariables (and thus suspected binaries), the known SB for which we lackinformation on the companion, the stars with an UV flux inconsistentwith their classification, and the fast rotating stars for which noaccurate abundance analysis can be performed, are also reviewed.Partly based on observations collected at the CFH Telescope (Hawaii) andat TBL of the Pic du Midi Observatory (France).Table \ref{tab5} is only available in electronic form athttp://www.edpsciences.org
|
Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
|
The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
|
How many lambda Bootis stars are binaries?
In the attempt to shed new light on the lambda Boo phenomenon weanalyzed the astrometric, photometric and spectroscopic characteristicsof stars out of a list of recently selected lambda Boo candidates. Weshow that the class is still ill-defined and discuss the possibilitythat some, if not most stars presently classified as lambda Boo, are infact binary pairs and that peculiar abundances may not correspond toactual values if the average values of the atmospheric parameters{Teff} and log g are assumed and the effect of veiling is nottaken into account. Partly based on data from the ESA Hipparcosastrometry satellite.
|
Photometry from the HIPPARCOS Catalogue: Constant MCP Stars, Comparison and Check Stars
Photometry from the Hipparcos catalogue is used to verify the constancyof four magnetic CP stars, as well as the comparison and the check starsused for variability studies of normal and chemically peculiar B and Astars with the Four College Automated Photoelectric Telescope;variability in these stars can produce spurious results. A few of thecomparison stars are found to be variable and should be replaced forfuture differential photometric studies.
|
New lambda Bootis stars with a shell
We publish here the second part of our spectroscopic survey at highdispersion of some known and suspected lambda Bootis stars with a viewto detecting circumstellar shell features. Eight stars of our sampleexhibit such features. These stars are fast rotators, a result which isin line with Hohlweger and Rentzsch-Holm's study (1995). The analysis ofthe photometric data has allowed us to confirm the exclusion of a fewstars misclassified from the lambda Bootis group.
|
The Angular Momentum of Main Sequence Stars and Its Relation to Stellar Activity
Rotational velocities are reported for intermediate-mass main sequencestars it the field. The measurements are based on new, high S/N CCDspectra from the Coudé Feed Telescope of the Kitt Peak NationalObservatory. We analyze these rotation rates for a dependence on bothmass and age. We compare the average rotation speeds of the field starswith mean velocities for young stars in Orion, the Alpha Persei cluster,the Pleiades, and the Hyades. The average rotation speeds of stars moremassive than $\sim1.6$ \msun\experience little or no change during theevolutionary lifetimes of these stars on the zero age main sequence orwithin the main sequence band. Less massive stars in the range betwee n1.6\msun\ and 1.3\msun\ also show little decline in mean rotation ratewhile they are on the main sequence, and at most a factor of 2 decreasein velocity as they evolve off the main sequence. The {\it e}-foldingtime for the loss of angular momentum b y the latter group of stars isat least 1--2 billion years. This inferred characteristic time scale forspindown is far longer than the established rotational braking time forsolar-type stars with masses below $\sim1.3$ \msun. We conclude from acomparison of the trends in rotation with trends in chromospheric andcoronal activity that the overall decline in mean rotation speed alongthe main sequence, from $\sim2$ \msun\ down to $\sim1.3$ \msun, isimposed during the pre-main sequence phase of evolution, and that thispattern changes little thereafter while the star resides on the mainsequence. The magnetic activity implicated in the rotational spindown ofthe Sun and of similar stars during their main sequence lifetimes mus ttherefore play only a minor role in determining the rotation rates ofthe intermediate mass stars, either because a solar-like dynamo is weakor absent, or else the geometry of the magnetic field is appreciablyless effective in removing angular momentu m from these stars. (SECTION:Stars)
|
Hot Inner Disks that Appear and Disappear around Rapidly Rotating A-Type Dwarfs
At any one time, approximately one-quarter of the most rapidly rotatingnormal A-type dwarfs (V sin i >= 200 km s-1) show shell lines of TiII in the near-ultraviolet. Our observations during 22 years show thatthe lines appear and disappear on timescales of decades but do notdisplay significant changes within 1 year. This implies that they arenot remnants of the star formation but rather are probably caused bysporadic mass-loss events. A working hypothesis is that all A-type starsthat are rotating near their limits have these shells, but for onlyone-quarter of the time. Because these lines do not appear in stars withsmaller sin i, the shells must be disks. These are hot inner disks thatmay or may not be related to the cool outer disks seen by Smith andTerrile around beta Pic or through infrared excesses around Vega andother A-type dwarfs. The similar, limited line widths indicate that thedisks are ~7 R* above the stellar surfaces.
|
The photoelectric astrolabe catalogue of Yunnan Observatory (YPAC).
The positions of 53 FK5, 70 FK5 Extension and 486 GC stars are given forthe equator and equinox J2000.0 and for the mean observation epoch ofeach star. They are determined with the photoelectric astrolabe ofYunnan Observatory. The internal mean errors in right ascension anddeclination are +/- 0.046" and +/- 0.059", respectively. The meanobservation epoch is 1989.51.
|
The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS...99..135A&db_key=AST
|
Ca II H and K Filter Photometry on the UVBY System. II. The Catalog of Observations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2828T&db_key=AST
|
Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
|
Photometry of Stars in the Field of V345 and V553 Aquilae
Not Available
|
Ca II H and K filter photometry on the UVBY system. I - The standard system
A fifth filter (fwhm = 90 A) centered on Ca II H and K has beendeveloped for use with the standard uvby system. The filter, called Ca,is designed primarily for applications to metal-poor dwarfs and redgiants, regions where the uvby metallicity index, m(l), loses somesensitivity. An index, hk, is defined by replacing v in m(l) by Ca. Theeffects of interstellar extinction on the index are modeled anddemonstrated to be modest and relatively insensitive to spectral type.Observations of V, (b-y), and hk for 163 primary standards are detailedand transformed to the standard V and (b-y) system. A qualitativeanalysis using only the primary standards indicates that hk is moresensitive than m(l) over the regions of interest by about a factor of 3.
|
Photoelectric photometry of chemically peculiar stars at the Catania astrophysical observatory. I - Observatory of HD 2453, HD 71866, HD 72968 and HD 126515
Photoelectric observations of four CP stars are presented which werecarried out from 1969 through 1975 at the Catania AstrophysicalObservatory in the UBV natural system. For the star HD 2453 a revisedvalue (546.87) of the period is derived from the magnetic field dataavailable in the literature which satisfies the photometricobservations. The periods of the stars HD 71866 and HD 72968 areconfirmed to be 6.80054 and 11.305 days, respectively. For the longperiod variable HD 126515 Preston's 130.0 days value, deduced from themagnetic field variations, is shown to be the period of the lightvariations too.
|
The late A-type stars - Refined MK classification, confrontation with Stromgren photometry, and the effects of rotation
The MK spectral classification for late A-type stars is refined and theeffects of rotation of spectral classification and uvby(beta) photometryfor these stars are examined. It is found that, for A3 stars, the4417/4481 A wavelength ratio produces results that are inconsistent withthe Stark broadening of the H lines. It is suggested that this ratio isnot useful as a luminosity criterion at any spectral type.Self-consistent sequences of narrow- and broadline standards areestablished. The results of the refined classification system arecompared with Stromgren photometry, showing a set of low-v sin i A-typestars with anomalously large delta(c1) indices for theirluminosity types. It is proposed that these stars are rapid rotatorsseen at fairly low inclination angles.
|
The broad lambda-5200 continuum depression in the CP star HD 126515
Delta-a photometry of HD 126515 shows the extraordinary variability ofits continuum depression around 5200 A. The Delta-a peculiarity is about0.06 m over most of the rotation cycle, but drops to about half thatvalue at phase of minimum surface field strength Hs and minimum lineblocking. It is argued that the surface region which produces the highpeculiarity values is associated with the regions of enhanced spectralline strength, rather than with local field strength.
|
Absolute luminosity calibration of Stroemgren's 'late group'
A statistical parallax method based on the principle of maximumlikelihood is used to calibrate absolute luminosities for samples ofcooler stars constituting the 'late group' defined by Stromgren (1966).The samples examined include 415 stars of all luminosity classes and asubset comprising 86 main-sequence stars. Linear calibration relationsinvolving the Stromgren beta, (b-y), and bracketted c1 indices arederived which yield mean absolute magnitudes with an accuracy of 0.09magnitude for the overall sample and 0.13 magnitude for themain-sequence subsample. Several second-order relations are considered,and the results are compared with Crawford's (1975) calibrations as wellas with mean absolute magnitudes obtained from trigonometric parallaxes.The possible effect of interstellar absorption on the calibrationrelations is also investigated.
|
Catalogue of early-type stars measured in a narrow-band photometric system
A compilation of the photoelectric measurements in the Barbier-Morguleffsystem is presented. The catalogue includes data for 773 stars ofspectral type 08 to F6. 706 stars have been measured at least twice.
|
Rotation and shell spectra among A-type dwarfs.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973ApJ...182..809A&db_key=AST
|
Rotation of evolving A and F stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972A&A....18..428D&db_key=AST
|
Photoelectric observations of magnetic stars. III. HD 124224, HD 140160 and HD 224801.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971AJ.....76..630B&db_key=AST
|
INHABITED SKY
